Isothermal pellet reactor

In order to study the catalyst deactivation phenomenon under supercritical conditions and the difference between the liquid phase (LP) and supercritical fluid phase (SCFP) reactions, experiments were carried out in an isothermal tubular reactor (D=I2 mm, L=600 mm) packed with grounded Y-type zeolite pellets of 60 mesh. The experimental equipment for the LP and SCF reaction processes is illustrated in Figure 1. 

The isothermal diflhision reactor provides a means for measuring the composition of the reaction mixture at the center of the pellet and near the external face. The reactor is designed to... 

Naphtalene 104 Natural gas 342-364 Nitrogen 94-100, 327-328 Non-isothermal pellet 148-151, 153-162 reactor 414... 

The reactor point effectiveness can now be readily determined from the overall pellet effectiveness developed earlier together with the known time dependence of the change of active surface area. Under the assumptions of negligible external mass transfer resistance and an isothermal pellet, the reactor point effectiveness is simply the pellet effectiveness multiplied by (ks/ki,), where ks and kj, are the rate constants evaluated at the pellet surface and bulk-fluid temperatures ... 

The basic approximations made in arriving at the reactor point effectiveness are (1) isothermal pellet, (2) negligible external mass transfer resistance, and (3) estimation of the pellet center concentration by a simple relationship when the reaction is not severely diffusion-limited. The first two approximations are quite adequate in view of the fact that the mass Biot number is of the order of hundreds under realistic reaction conditions. Both theoretical and experimental justifications for these approximations have been given in Chapter 4. The first approximation will be relaxed when reactions affected by pore-mouth poisoning are considered since a definite temperature gradient then exists within the pellet. An additional approximation is the representation of the difference between the Arrhenius exponentials evaluated at the pellet surface and the bulk-fluid temperatures by a linear rela-... 

Consider a simple trickle-bed for which one can assume uniform gas and liquid distributions, a nonvolatile liquid, trickling flow, isothermal pellets, and complete wetting. For an isothermal reactor with a uniform distribution of the gas and liquid phases, there will be no radial gradients of concentration. Although axial dispersion is more important in trickle-beds than in fixed-beds because of relatively low fluid velocities, we will assume that the dispersion is negligible. Under the assumptions, one can write for the gas phase (refer to Figure 12.3) ... 

Catalyst Effectiveness. Even at steady-state, isothermal conditions, consideration must be given to the possible loss in catalyst activity resulting from gradients. The loss is usually calculated based on the effectiveness factor, which is the diffusion-limited reaction rate within catalyst pores divided by the reaction rate at catalyst surface conditions (50). The effectiveness factor E, in turn, is related to the Thiele modulus,

first-order rate constant, a the internal surface area, and the effective diffusivity. It is desirable for E to be as close as possible to its maximum value of unity. Various formulas have been developed for E, which are particularly usehil for analyzing reactors that are potentially subject to thermal instabilities, such as hot spots and temperature mnaways (1,48,51). 

Suppose that catalyst pellets in the shape of right-circular cylinders have a measured effectiveness factor of r] when used in a packed-bed reactor for a first-order reaction. In an effort to increase catalyst activity, it is proposed to use a pellet with a central hole of radius i /, < Rp. Determine the best value for RhjRp based on an effective diffusivity model similar to Equation (10.33). Assume isothermal operation ignore any diffusion limitations in the central hole, and assume that the ends of the cylinder are sealed to diffusion. You may assume that k, Rp, and eff are known. 

Barnett et al. [AIChE J., 7 (211), 1961] have studied the catalytic dehydrogenation of cyclohexane to benzene over a platinum-on-alumina catalyst. A 4 to 1 mole ratio of hydrogen to cyclohexane was used to minimize carbon formation on the catalyst. Studies were made in an isothermal, continuous flow reactor. The results of one run on 0.32 cm diameter catalyst pellets are given below. 

A first-order chemical reaction occurs isothermally in a reactor packed with spherical catalyst pellets of radius R. If there is a resistance to mass transfer from the main fluid stream to the surface of the particle in addition to a resistance within the particle, show that the effectiveness factor for the pellet is given by ... 

In practice, of course, it is rare that the catalytic reactor employed for a particular process operates isothermally. More often than not, heat is generated by exothermic reactions (or absorbed by endothermic reactions) within the reactor. Consequently, it is necessary to consider what effect non-isothermal conditions have on catalytic selectivity. The influence which the simultaneous transfer of heat and mass has on the selectivity of catalytic reactions can be assessed from a mathematical model in which diffusion and chemical reactions of each component within the porous catalyst are represented by differential equations and in which heat released or absorbed by reaction is described by a heat balance equation. The boundary conditions ascribed to the problem depend on whether interparticle heat and mass transfer are considered important. To illustrate how the model is constructed, the case of two concurrent first-order reactions is considered. As pointed out in the last section, if conditions were isothermal, selectivity would not be affected by any change in diffusivity within the catalyst pellet. However, non-isothermal conditions do affect selectivity even when both competing reactions are of the same kinetic order. The conservation equations for each component are described by... 

Our reaction A R proceeds isothermally in a packed bed of large, slowly deactivating catalyst particles and is performing well in the strong pore diffusion regime. With fresh pellets conversion is 88% however, after 250 days conversion drops to 64%. How long can we run the reactor before conversion drops to... 

Kinetic experiments were carried out isothermally in autoclave reactors of sizes 500 ml and 600 ml. The stirring rate was typically 1500 rpm. In most cases, the reactors operated as slurry reactors with small catalyst particles (45-90 micrometer), but comparative experiments were carried out with a static basket using large trilobic catalyst pellets (citral hydrogenation). Samples were withdrawn for analysis (GC for citral hydrogenation and HPLC for lactose hydrogenation). The experimental details as well as qualitative kinetics are reported in previous papers of our group Kuusisto et al. (17), Aumo et al. (5). 

Critical effects in CO oxidation over Pt catalysts were obtained [33, 34, 63-85] in various catalytic systems over wires, foils and gauzes, on single pellets and fixed beds, in isothermal and adiabatic reactors (differential and integral). The literature also reported the oscillating behaviour of the homogeneous oxidation of CO [86, 87]. 

The kinetic experiments, activity tests, and poisoning experiments were carried out in a gas-flow isothermal fixed bed reactor [6) at the benzene partial pressure of 7.55 kPa hydro gen partial pressure 99.82 kPa thiophene partial pressure 0.032 kPa and the reaction temperatures 403, 427 and 448 K. The size of the commercial cylindrical catalyst pellet was 5x5mm (21% Ni on alumina, supplied by BASF). The nickel oxide containing precursor was activated by reduction with hydrogen at 743 K for 10 hr. 

Figure 19. A) Design concept of the Linde isothermal reactor for methanol synthesis B) Cut through the tube bundle surrounded by catalyst pellets (from [26]).

The liquid is initially distributed by means of four small 1/16" pipes. An inert bed, realized with inert alumina pellets, ensured preheating, liquid saturation and an uniform distribution of both fluids. The reactor is provided with an axial thermocouple well. A sliding thermocouple can be moved along the bed axis allowing to measure the axial temperature profile and to check the isothermal operation of the reactor. 

Figure 2 (a) Deactivation of Pt/silica (D) in isothermal cyclohexene hydrogenation under conditions mentioned in the text in the reactor shown in (b). (c) Ruidised bed reactor in which entered at 1 and passed by 1.7%Pt/alumina pellets and fluidised 3g silica-alumina powder thereon (which also received QHiq/Nj entering at a low rate at 2) in which the activity of the silica-alumina shown in (d) was recorded at 323K. These data were measured in the absence (O ) and the presence ( C first run 9 second run) of the Pt/alumina pellets, and after the removal of these pellets ( ). The dependence of the activity (d) of this silica-alumina on the weight of pellets used in retesting is shown in (e)... 

A considerable amount of experimental information has been obtained for difiusion-reaction-deactivation in pellets by the use of so-called "single pellet difiiision reactors". These come in two forms the first, origiiMy developed by Balder and Petersen [11] ("Petersen s Pellet Poisoner"), relies upon the analysis of concentrations of reactants and their variation with time the second, fi om Kehoe and Butt [12] ("Kehoe s Katalyst Killer") involves measurement of temperature profiles within the pellet and their variation with time. The first is usefiil for deactivation in isothermal systems the second, at the expense of more complexity, can be used for both isothermal and nonisothermal systems. It is not possible to go into a long discussion of these systems here, but some discussion of the more simple reactor, for isothermal systems, will be useful as an example. 

Isothermal effectiveness factors for practical reactions cover a wide range, from as low as 0.01 to unity. With normal pellet sizes (I to y in.) r] is 0.7 to 1.0 for intrinsically slow reactions, such as the ammonia synthesis, and of the order of t/ = 0.1 for fast reactions, such as some hydrogenations of unsaturated hydrocarbons. Satterfield and Sherwood have summarized much of the experimental data for effectiveness factors for various reactions, temperatures, and pellet sizes. For reactor design it is important to be able to answer these questions ... 

In the laboratory either integral or differential (see Sec. 4-3) tubular units or stirred-tank reactors may be used. There are advantages in using stirred-tank reactors for kinetic studies. Steady-state operation with well-defined residence-time conditions and uniform concentrations in the fluid and on the solid catalyst are achieved. Isothermal behavior in the fluid phase is attainable. Stirred tanks have long been used for homogeneous liquid-phase reactors and slurry reactors, and recently reactors of this type have been developed for large catalyst pellets. Some of these are described in Sec. 12-3. When either a stirred-tank or a differential reactor is employed, the global rate is obtained directly, and the analysis procedure described above can be initiated immediately. 

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